1. Field of the Invention
The present invention relates to a thermally fusible composite fiber, and to non-woven fabric made of such a fiber.
2. Description of the Prior Art
A low-density non-woven fabric of a METSUKE (weight per unit area) between approximately 10 and approximately 45 g/m.sup.2 is used as the surface material for paper diapers, sanitary napkins, and the like. As the uses of non-woven fabrics have become diversified, property requirements for non-woven fabrics have become more strict, and there has been demand for non-woven fabrics which maintain high strength at a minimum weight while retaining a soft texture. In the context of such a recent situation, products such as pants-type diapers are required to have a certain strength, and this is accomplished by heat-sealing non-woven fabrics with each other. For this reason, a non-woven fabric having excellent heat-sealing properties is demanded.
In order to satisfy such a demand, it is necessary that the non-woven fabric be constituted of fine, thermally fusible composite fibers, and that the low-melting component contributing to the thermal fusion of thermally fusible composite fibers have sufficient adhesive strength as well as flexibility.
Examples of thermally fusible composite fibers include the combinations of polypropylene and polyethylene, polyethylene terephthalate and polyethylene, and polyethylene terephthalate and poly(ethylene terephthalate)-co-(ethylene isophthalate). The polyethylene materials include high-density polyethylene, low-density polyethylene, and linear low-density polyethylene.
However, when low-density polyethylene or linear low-density polyethylene is used as the low-melting component of the thermally fusible fibers, the fibers may become adhered to one another at a low temperature, but are easily peeled apart. Also, although the resultant non-woven fabric has a soft feel, it has low strength, low rigidity due to low density, and a sticky feel. For example, Japanese Patent Application Laid-Open No. 63-92722 discloses a. fine thermally fusible composite fiber using linear low-density polyethylene having a low rigidity as the low-melting component, as well as a thermally fusible non-woven fabric comprising such a fiber. However, this fabric has poor heat-sealing properties and a low strength, and does not satisfy the requirements of the non-woven fabric achieving the object of the present invention.
On the other hand, non-woven fabric made of thermally fusible composite fibers in which high-density polyethylene is used as the low-melting component has higher density and rigidity, higher strength, and good heat-sealing properties as compared to non-woven fabrics made of low-density polyethylene and linear low-density polyethylene. However, since the high-density polyethylene used as the low-melting component has a high melting point, the processing temperature must be elevated in order to achieve sufficient non-woven strength and heat-sealing properties. This is disadvantageous in that the resultant non-woven fabric has a stiff feel. Furthermore, although lower non-woven processing temperatures are desirable from the point of view of energy costs, sufficient strength cannot be achieved unless the processing temperature is sufficiently high.
In order to solve such problems, a thermally fusible composite fiber disclosed in Japanese Patent Application Laid-Open No. 2-251612 has as its high-melting component polypropylene or polyester, and as its low-melting component high-density polyethylene, which has many methyl branches in its molecular chain and a relatively low melting point. However, increasing the number of methyl branches in polyethylene generally lowers the density, and increasing the Q value (weight average molecular weight Mw/number average molecular weight Mn) increases the unevenness of the polymer. Both of these effects lower the tensile strength of the low-melting component, lower the adhesive strength of the low-melting component at points where fibers intersect one another, and result in insufficient strength of the fabric itself and of heat sealing.